API: enforce sha requirement on POST /repos/{owner}/{repo}/contents (#8139)

Currently the `POST /repos/{owner}/{repo}/contents` API endpoint accepts request without any `ChangeFileOperation.SHA`, unlike stated by the doc:
33eee199cf/modules/structs/repo_file.go (L80-L81)

This PR adds:
- some more (already passing) tests around this function
- a new (failing) test to show this wrong behavior
- a fix (note that this is a breaking change for clients exploiting this bug)
- an update for all the existing tests

<!--start release-notes-assistant-->

## Release notes
<!--URL:https://codeberg.org/forgejo/forgejo-->
- Breaking bug fixes
  - [PR](https://codeberg.org/forgejo/forgejo/pulls/8139): <!--number 8139 --><!--line 0 --><!--description QVBJOiBlbmZvcmNlIHNoYSByZXF1aXJlbWVudCBvbiBgUE9TVCAvcmVwb3Mve293bmVyfS97cmVwb30vY29udGVudHNg-->API: enforce sha requirement on `POST /repos/{owner}/{repo}/contents`<!--description-->
<!--end release-notes-assistant-->

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8139
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: oliverpool <git@olivier.pfad.fr>
Co-committed-by: oliverpool <git@olivier.pfad.fr>

tests/e2e/declare_repos_test.go

@@ -74,6 +74,7 @@ func newRepo(t *testing.T, userID int64, repoName string, fileChanges []FileChan
 		nil,
 	)
 
+	var lastCommitID string
 	for _, file := range fileChanges {
 		for i, version := range file.Versions {
 			operation := "update"
@@ -108,9 +109,12 @@ func newRepo(t *testing.T, userID int64, repoName string, fileChanges []FileChan
 					Author:    time.Now(),
 					Committer: time.Now(),
 				},
+				LastCommitID: lastCommitID,
 			})
 			require.NoError(t, err)
 			assert.NotEmpty(t, resp)
+
+			lastCommitID = resp.Commit.SHA
 		}
 	}